The present invention relates to a process for cracking an olefin-rich hydrocarbon feedstock which is selective towards light olefins in the effluent. In particular, olefinic feedstocks from refineries or petrochemical plants can be converted selectively so as to redistribute the olefin content of the feedstock in the resultant effluent. More particularly, the present invention relates to such a process which is resistant to impurities contained in the feedstock.
It is known in the art to use zeolites to convert long chain paraffins into lighter products, for example in the catalytic dewaxing of petroleum feedstocks. While it is not the objective of dewaxing, at least parts of the paraffinic hydrocarbons are converted into olefins. It is known in such processes to use crystalline silicates for example of the MFI type, the three-letter designation xe2x80x9cMFIxe2x80x9d representing a particular crystalline silicate structure type as established by the Structure Commission of the International Zeolite Association. Examples of a crystalline silicate of the MFI type are the synthetic zeolite ZSM-5 and silicalite and other MFI type crystalline silicates are known in the art.
GB-A-1323710 discloses a dewaxing process for the removal of straight-chain paraffins and slightly branched-chain paraffins, from hydrocarbon feedstocks utilising a crystalline silicate catalyst, in particular ZSM-5. U.S. Pat. No. 4,247,388 also discloses a method of catalytic hydrodewaxing of petroleum and synthetic hydrocarbon feedstocks using a crystalline silicate of the ZSM-5 type. Similar dewaxing processes are disclosed in U.S. Pat. No. 4,284,529 and U.S. Pat. No. 5,614,079. The catalysts are crystalline alumino- silicates and the above-identified prior art documents disclose the use of a wide range of Si/Al ratios and differing reaction conditions for the disclosed dewaxing processes.
GB-A-2185753 discloses the dewaxing of hydrocarbon feedstocks using a silicalite catalyst. U.S. Pat. No. 4,394,251 discloses hydrocarbon conversion with a crystalline silicate particle having an aluminum-containing outer shell.
It is also known in the art to effect selective conversion of hydrocarbon feeds containing straight-chain and/or slightly branched-chain hydrocarbons, in particular paraffins, into a lower molecular weight product mixture containing a significant amount of olefins. The conversion is effected by contacting the feed with a crystalline silicate known as silicalite, as disclosed in GB-A-2075045, U.S. Pat. No. 4,401,555 and U.S. Pat. No. 4,309,276. Silicalite is disclosed in U.S. Pat. No. 4,061,724.
Silicalite catalysts exist having varying silicon/aluminum atomic ratios and different crystalline forms. EP-A-0146524 and 0146525 in the name of Cosden Technology, Inc. disclose crystalline silicas of the silicalite type having monoclinic symmetry and a process for their preparation. These silicates have a silicon to aluminum atomic ratio of greater than 80.
WO-A-97/04871 discloses the treatment of a medium pore zeolite with steam followed by treatment with an acidic solution for improving the butene selectivity of the zeolite in catalytic cracking.
A paper entitled xe2x80x9cDe-alumination of HZSM-5 zeolites: Effect of steaming on acidity and aromatization activityxe2x80x9d, de Lucas et al, Applied Catalysis A: General 154 1997 221-240, published by Elsevier Science B.V. discloses the conversion of acetone/n-butanol mixtures to hydrocarbons over such dealuminated zeolites.
It is yet further known, for example from U.S. Pat. No. 4,171,257, to dewax petroleum distillates using a crystalline silicate catalyst such as ZSM-5 to produce a light olefin fraction, for example a C3 to C4 olefin fraction. Typically, the reactor temperature reaches around 500xc2x0 C. and the reactor employs a low hydrocarbon partial pressure which favours the conversion of the petroleum distillates into propylene. Dewaxing cracks paraffinic chains leading to a decrease in the viscosity of the feedstock distillates, but also yields a minor production of olefins from the cracked paraffins.
EP-A-0305720 discloses the production of gaseous olefins by catalytic conversion of hydrocarbons. EP-B-0347003 discloses a process for the conversion of a hydrocarbonaceous feedstock into light olefins. WO-A-90/11338 discloses a process for the conversion of C2-C12 paraffinic hydrocarbons to petrochemical feedstocks, in particular to C2 to C4 olefins. U.S. Pat. No. 5,043,522 and EP-A-0395345 disclose the production of olefins from paraffins having four or more carbon atoms. EP-A-0511013 discloses the production of olefins from hydrocarbons using a steam activated catalyst containing phosphorous and H-ZSM-5. U.S. Pat. No. 4,810,356 discloses a process for the treatment of gas oils by dewaxing over a silicalite catalyst. GB-A-2156845 discloses the production of isobutylene from propylene or a mixture of hydrocarbons containing propylene. GB-A-2159833 discloses the production of a isobutylene by the catalytic cracking of light distillates.
It is known in the art that for the crystalline silicates exemplified above, long chain olefins tend to crack at a much higher rate than the corresponding long chain paraffins.
It is further known that when crystalline silicates are employed as catalysts for the conversion of paraffins into olefins, such conversion is not stable against time. The conversion rate decreases as the time on stream increases, which is due to formation of coke (carbon) which is deposited on the catalyst.
These known processes are employed to crack heavy paraffinic molecules into lighter molecules. However, when it is desired to produce propylene, not only are the yields low but also the stability of the crystalline silicate catalyst is low. For example, in an FCC unit a typical propylene output is 3.5 wt %. The propylene output may be increased to up to about 7-8 wt % propylene from the FCC unit by introducing the known ZSM-5 catalyst into the FCC unit to xe2x80x9csqueezexe2x80x9d out more propylene from the incoming hydrocarbon feedstock being cracked. Not only is this increase in yield quite small, but also the ZSM-5 catalyst has low stability in the FCC unit.
There is an increasing demand for propylene in particular for the manufacture of polypropylene.
The petrochemical industry is presently facing a major squeeze in propylene availability as a result of the growth in propylene derivatives, especially polypropylene. Traditional methods to increase propylene production are not entirely satisfactory. For example, additional naphtha steam cracking units which produce about twice as much ethylene as propylene are an expensive way to yield propylene since the feedstock is valuable and the capital investment is very high. Naphtha is in competition as a feedstock for steam crackers because it is a base for the production of gasoline in the refinery. Propane dehydrogenation gives a high yield of propylene but the feedstock (propane) is only cost effective during limited periods of the year, making the process expensive and limiting the production of propylene. Propylene is obtained from FCC units but at a relatively low yield and increasing the yield has proven to be expensive and limited. Yet another route known as metathesis or disproportionation enables the production of propylene from ethylene and butene. Often, combined with a steam cracker, this technology is expensive since it uses ethylene as a feedstock which is at least as valuable as propylene.
EP-A-0109059 discloses a process for converting olefins having 4 to 12 carbon atoms into propylene. The olefins are contacted with an alumino-silicate having a crystalline and zeolite structure (e.g. ZSM-5 or ZSM-11) and having a SiO2/Al2O3 molar ratio equal to or lower than 300. The specification requires high space velocities of greater than 50 kg/h per kg of pure zeolite in order to achieve high propylene yield. The specification also states that generally the higher the space velocity the lower the SiO2/Al2O3 molar ratio (called the Z ratio). This specification only exemplifies olefin conversion processes over short periods (e.g. a few hours) and does not address the problem of ensuring that the catalyst is stable over longer periods (e.g. at least 160 hours or a few days) which are required in commercial production. Moreover, the requirement for high space velocities is undesirable for commercial implementation of the olefin conversion process.
Thus there is a need for a high yield propylene production method which can readily be integrated into a refinery or petrochemical plant, taking advantage of feedstocks that are less valuable for the market place (having few alternatives on the market).
On the other hand, crystalline silicates of the MFI type are also well known catalysts for the oligomerisation of olefins. For example, EP-A-0031675 discloses the conversion of olefin-containing mixtures to gasoline over a catalyst such as ZSM-5. As will be apparent to a person skilled in the art, the operating conditions for the oligomerisation reaction differ significantly from those used for cracking. Typically, in the oligomerisation reactor the temperature does not exceed around 400xc2x0 C. and a high pressure favours the oligomerisation reactions.
GB-A-2156844 discloses a process for the isomerisation of olefins over silicalite as a catalyst. U.S. Pat. No. 4,579,989 discloses the conversion of olefins to higher molecular weight hydrocarbons over a silicalite catalyst. U.S. Pat. No. 4,746,762 discloses the upgrading of light olefins to produce hydrocarbons rich in C5+ liquids over a crystalline silicate catalyst. U.S. Pat. No. 5,004,852 discloses a two-stage process for conversion of olefins to high octane gasoline wherein in the first stage olefins are oligomerised to C5+ olefins. U.S. Pat. No. 5,171,331 discloses a process for the production of gasoline comprising oligomerising a C2-C6 olefin containing feedstock over an intermediate pore size siliceous crystalline molecular sieve catalyst such as silicalite, halogen stabilised silicalite or a zeolite. U.S. Pat. No. 4,414,423 discloses a multistep process for preparing high-boiling hydrocarbons from normally gaseous hydrocarbons, the first step comprising feeding normally gaseous olefins over an intermediate pore size siliceous crystalline molecular sieve catalyst. U.S. Pat. No. 4,417,088 discloses the dimerising and trimerising of high carbon olefins over silicalite. U.S. Pat. No. 4,417,086 discloses an oligomerisation process for olefins over silicalite. GB-A-2106131 and GB-A-2106132 disclose the oligomerisation of olefins over catalysts such as zeolite or silicalite to produce high boiling hydrocarbons. GB-A-2106533 discloses the oligomerisation of gaseous olefins over zeolite or silicalite.
It is known that hydrocarbon feedstocks can contain impurities including nitrogen, oxygen and sulphur heteroatoms. Such impurities act as poisons for crystalline silicate catalysts, thus reducing the catalyst activity and product yield over time. There is a need for crystalline silicate catalysts coupled with selected process conditions which are resistant to such impurities, leading to the opportunity to use a variety of feedstocks of varying purity in the hydrocarbon conversion process.
It is an object of the present invention to provide a process for using the less valuable olefins present in refinery and petrochemical plants as a feedstock for a process which, in contrast to the prior art processes referred to above, catalytically converts olefins into lighter olefins, and in particular propylene.
It is also an object of the invention to provide such a process wherein the olefin feedstock contains impurities, in particular sulphur-, nitrogen- and oxygen-derivative containing impurities.
It is another object of the invention to provide a process for producing propylene having a high propylene yield and purity.
It is a further object of the present invention to provide such a process which can produce olefin effluents which are within, at least, a chemical grade quality.
It is yet a further object of the present invention to provide a process for producing olefins having a stable olefinic conversion and a stable product distribution over time.
It is yet a further object of the present invention to provide a process for converting olefinic feedstocks having a high yield on an olefin basis towards propylene, irrespective of the origin and composition of the olefinic feedstock.
It is still a further object of the invention to provide a process for olefin catalytic cracking wherein the catalyst has high stability, for example capable of giving a stable olefin yield over a significant period of time, typically several days.
It is another object of the invention to provide a catalytic cracking process employing such a catalyst which has high flexibility so that it can operate with a variety of different feedstocks, which may be mixtures.
The present invention provides a process for the catalytic cracking of at least one olefin in an olefinic stream containing impurities, the cracking process being selective towards light olefins in the effluent, the process comprising contacting a feedstock olefinic stream containing at least one sulphur-, nitrogen- and/or oxygen-derivative impurity with a crystalline silicate catalyst of the MFI-type, the catalyst having a silicon/aluminum atomic ratio of at least about 180, to produce an effluent stream having substantially the same olefinic content by weight as, but a different olefin distribution than, the feedstock stream.
The present invention can thus provide a process wherein olefin-rich hydrocarbon streams (products) from refinery and petrochemical plants are selectively cracked not only into light olefins, but particularly into propylene. In one preferred embodiment the olefin-rich feedstock may be passed over a crystalline silicate catalyst with a particular Si/Al atomic ratio of from 180 to 1000 obtained after a steaming/de-alumination treatment. Alternatively the olefin-rich feedstock may be passed over a commercially available catalyst of the ZSM-5 type which has been prepared by crystallisation using an organic template and has been unsubjected to any subsequent steaming or de-alumination process, the catalyst having a silicon/aluminum atomic ratio of from 300 to 1000. The feedstock may be passed over the catalyst at a temperature ranging between 500 to 600xc2x0 C., an olefin partial pressure of from 0.1 to 2 bars and an LHSV of from 10 to 30hxe2x88x921 to yield at least 30 to 50% propylene based on the olefin content in the feedstock.
In this specification, the term xe2x80x9csilicon/aluminum atomic ratioxe2x80x9d is intended to mean the Si/Al atomic ratio of the overall material, which may be determined by chemical analysis. In particular, for crystalline silicate materials, the stated Si/Al ratios apply not just to the Si/Al framework of the crystalline silicate but rather to the whole material.
The silicon/aluminum atomic ratio is greater than about 180. Even at silicon/aluminum atomic ratios less than about 180, the yield of light olefins, in particular propylene, as a result of the catalytic cracking of the olefin-rich feedstock may be greater than in the prior art processes. The feedstock may be fed either undiluted or diluted with an inert gas such as nitrogen. In the latter case, the absolute pressure of the feedstock constitutes the partial pressure of the hydrocarbon feedstock in the inert gas.